Coating is the process of applying a layer of fluid, typically referred to as a coating material or coating solution, to a substrate. The substrate may be provided in many forms, but is often provided in the form of a long continuous sheet of material wound into a roll, commonly referred to as a web. Typical substrate materials include plastic film, woven or non-woven fabric, or paper. One method of coating the substrate involves unwinding the web from the supply roll, applying a liquid layer of coating material to the web, solidifying the liquid layer on the web, and rewinding the coated web into a roll.
After the coating material is applied, it can remain a liquid such as in the application of lubricating oil to metal in metal coil processing or the application of chemical reactant to activate or chemically transform a substrate surface. Alternatively, the coating material can be solidified by drying if it contains a volatile liquid, or can be cured by heat, ultraviolet radiation, or the like, or treated in some other way to leave behind a solid coated layer. Examples of typical coating materials include paints, varnishes, adhesives, photochemicals, and magnetic recording media. Methods of applying coatings to webs are discussed in Cohen, E. D. and Gutoff, E. B., Modern Coating and Drying Technology, VCH Publishers, New York 1992 and Satas, D., Web Processing and Converting Technology and Equipment, Van Vortstrand Reinhold Publishing Co., New York 1984, and include knife coaters.
Knife coating involves passing the coating material between a stationary solid member, a knife, and the web so that the clearance between the knife and the web is less than twice the thickness of the applied coating material. The coating material is sheared between the web and the knife, and the thickness of the applied coating material depends to a great extent on the height of the clearance. Alternatively, knife coaters can apply a coating directly to a roller, which can subsequently transfer the coating to a web.
One feature which distinguishes various knife coaters is how the coating material is supplied to the clearance between the knife and the web. Some different types of knife coaters include die-fed knife coaters and trough-fed knife coaters such as a cross flow knife coater. With each of these types of coaters, the coating material may be applied across the entire transverse width of the web, or may be applied in stripes or zones across the width of the web.
Die-fed knife coaters, as illustrated in FIG. 1, receive coating material from a narrow slot which, in conjunction with an upstream manifold, distributes evenly across the web the flow of coating material feeding the knifing passage. The die includes two plates sandwiched together with a shim or a depression in one plate forming the slot passage. In order to apply the coating material in stripes or zones with a die-fed knife coater, the slot passage is typically blocked at specific areas so that the coating material cannot exit the slot passage in those areas. The coating material can only exit the slot passage at the unblocked areas, thereby providing the desired pattern of coated and uncoated zones on the web. The slot passage may be blocked either by inserting shims into the slot passage or by covering specific parts of the slot passage, such as with a piece of tape or other covering material.
Trough-fed knife coaters, shown in FIGS. 2A and 2B, receive coating material from a wide slot, or trough, which is fed by a narrow slot and manifold to provide even flow distribution across the web. In order to apply the coating material in stripes or zones with a trough-fed knife coater, the trough is typically blocked at specific areas so that the coating material cannot exit the trough in those areas. The coating material can only exit the trough at the unblocked areas, thereby providing the desired pattern of coated and uncoated zones on the web. The trough may be blocked by covering specific parts of the trough, such as with a piece of tape or other covering material. However, because the web may contact with this tape as it moves past the trough during the coating process, undesirable scratching and damage of the web may occur.
Alternatively, the trough may be blocked by inserting dams into the trough, where each dam is the same width as the area to remain uncoated on the web. The sides of the dams correspond to the edges between coated and uncoated areas on the web and are parallel to the machine direction. When dams of this type are used, it is common for an edge bead of coating material to form at the sides of the dams, which then tends to flow onto the upper surface of the dam that is in contact with the web. Any coating material that has flowed onto the upper surface of the dam may then transfer to areas of the web that were to remain free of coating material. When this happens, the web product often will not meet the necessary manufacturing specifications and must be discarded.
Cross flow knife coaters, shown in FIG. 3, receive coating material from a wide slot, or trough, which is fed at one transverse end of the trough and flows across the width of the trough to the opposite transverse end of the trough to provide even flow distribution across the web. Any coating material that is not coated onto the web surface exits the end of the trough opposite the supply end. In order to apply coating material in stripes or zones with a cross flow knife coater, the trough is typically blocked at specific areas with tape or dams in the same manner as the trough-fed knife coater so that the coating material cannot exit the trough in those areas. As with the trough-fed knife coater, it is difficult to apply coating material in stripes or zones with a cross flow knife coater using the current methods of stripe coating.